Gorilla Crabs in Reef Tanks: Identification & Tips

Imagine watching your tank one day, only to find a swath of damaged corals around a cavity in the live rock. And imagine further your surprise when you see a hairy, cheeky little claw poke out to damage your corals further before retreating back into the aquascape! If this is something you don’t have to imagine then it’s likely you’re dealing with a Gorilla Crab in your reef tank, one of the most irritating pests you can accidentally end up with…

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What are Gorilla Crabs?

Gorilla Crabs are particularly hairy members of the family Xanthidae. But the names are interchangeable because there are over 500 different species of xanthid crabs formally described and likely hundreds more waiting to be discovered. Xanthid crabs are by far the most diverse family of crabs in terms of species richness!

Gorilla crabs get their name from the hair-like fuzz that covers their shells, legs, and claws. Just how much and what color the hair is depends on the species. Some can be entirely downy and covered in brown, tan, or light green hair while others may only have hair on their claws, legs, or almost none. Hairless versions tend to be called xanthid or stone crabs but know that they are all members of the family Xanthidae.

Many Xanthid Crabs, interestingly enough, are extremely poisonous as well. They have lethal concentrations of tetrodotoxin in their muscles and egg masses, enough to even kill you, should you be foolish enough to try eating one. While that’s not too likely, a fish nibbling on a dead crab will likely end up dead itself.

While they are very common, the majority of gorilla and xanthid crabs are fairly ugly and not interesting enough for most aquarists to purchase. So how do Gorilla Crabs get into your tank to begin with? And is it even a problem to have one – or several of them?

  • Common Names: Gorilla Crab, Xanthid Crab, Stone Crab
  • Scientific Name: several, including Pilumnus vespertillo. Family Xanthidae
  • Origin: Worldwide
  • Size: ½ to 2 inches
  • Temperament: Aggressive; Predatory
  • Reef-Safe: No

What’s Wrong with Gorilla Crabs in Reef Tanks?

Xanthid and Gorilla Crabs, when first discovered, will likely excite you! Crabs are some of the more active reef dwellers you’ll come across. And even if you have a fish-only setup, crabs are tough enough to live alongside any but the most determined crab-eaters, like Triggerfish, Pufferfish, and Wrasses. Crabs are also scavengers and detritivores that will eat any leftover food missed by your fish and assist your saltwater clean up crew in keeping things looking tidy!

The problem with Xanthid and Gorilla Crabs is that they won’t be content with eating just scraps and leftovers. Gorilla Crabs, and most crabs in general, are actually opportunistic predators. Meaning they’ll scavenge and eat whatever they can find – but they will gladly dine on fresher fare if they can get a hold of it and overpower it.

Gorilla Crabs think nothing of snacking on nearby coral polyps, feather duster worm fronds, nipping the tentacles off of an anemone, or even snatching a sleeping fish or one that blunders into its lair. Gorilla Crabs are usually quite small but they can overpower just about anything else in their size range or even a little larger. Many other crustaceans are also opportunists. But most saltwater shrimp, hermit crabs, and other safer crustaceans don’t have the weight, strength, or heavy claws to actively prey on their tank mates.

They can also be extremely difficult to remove from an aquarium. Once a Gorilla Crab finds a hiding spot they tend to stick to it, only coming out at night. They can scuttle quite quickly and should you try to dip a net in, they will dash straight back to their lair. What’s more, once they’ve taken a liking to a spot, they will actively scrape away at your live rock or coral, enlarging the lair enough to fit them more comfortably. This enlarging can destabilize your entire live rock structure from within, potentially causing partial or total collapses of your coral aquascape!

You might be thinking “well, if my Gorilla Crab causes problems, I can just remove it.” Removing a Gorilla Crab is often an exercise in frustration and typically ends with you having to remove the majority of your aquascape, piece by piece, in order to reach the crab. And if you’ve already superglued the substrate together, you’re going to have an even more frustrating time…

Do I Have a Gorilla Crab or Something Else?

The first question you should be asking is whether you even have Xanthid or Gorilla Crabs! While they are deadly poisonous, this defense is more of a last resort mechanism. Therefore the crab relies on its camouflage and ability to hide in small crevices to escape predators like you. In fact, you may not ever know that you have Gorilla Crab(s) for quite some time since they are usually also nocturnal.

The best way to tell is to examine your tank after dark or right as the aquarium lights come on. You may spot one or several scuttling away to their hiding places. A well established, fully grown Gorilla Crab may spend more time out in the open, hunting for morsels.

But at night you can search using a red flashlight, which is invisible to many marine animals. Red light is absorbed by the first few feet of seawater, so few animals have the ability to see light in this wavelength. This way, you can observe your crab in the act while also not disturbing the other aquarium inhabitants.

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During the day you may see damaged corals and other sessile invertebrates near the base of the crab’s lair. Sometimes a crab can get so comfortable that it can simply stick its claw out, snipping off a piece of coral flesh to munch on, without ever exposing itself to danger!

Disappearing fish can also signal a Gorilla Crab infestation. Since they are opportunists they will also eat weak, sick, or fish that are already dead. The crab might not be the one to kill your fish – but if you find that a fish has entirely vanished or has signs of being munched on, it’s possible a crab was responsible.

What’s the Difference Between Gorilla and Emerald Crabs?

Emerald Crabs (Mithraculus sculptus) are distant cousins and also very common live rock hitchhikers. They do have a strong resemblance to many Xanthid Crabs, so it’s worth talking about how to tell the difference. Because while Gorilla Crabs are often predatory, Emerald Crabs are mostly vegetarian. They spend their time scouring live rock for macro algae, hair algae, and other greenery to eat.

Emerald Crabs are never covered in hair and are typically some shade of green, from a dull forest green to a vibrant true emerald color. The colors of the claws are also different; Gorilla and Xanthid Crabs tend to have dark tips to the claws while Emerald Crabs have either a creamy color to the tip or are simply green, from shell to claw tips.

How Can I Remove a Gorilla Crab?

Gorilla Crabs, once established, are very difficult to remove. You can’t use chemical means because invertebrates tend to be broadly sensitive to a wide range of agents. Copper based remedies will certainly kill a Gorilla Crab – and also your shrimp, corals, and other creatures!

Gorilla Crab Spears

The two most frequently used and successful ways to remove a Gorilla Crab are to either spear them or kill them. Spearing a Gorilla Crab involves using a set of tweezers, chopsticks, coral propagation tools, or whatever else you have on hand to either crush or remove the crab from the aquarium.

If the crab is out in the open, you’ll have to be quick if you want to catch it. But it’s much more likely to be lurking in or near its lair, ready to make a quick getaway. The orientation and layout of the hole may still allow you to reach inside and pluck the crab out. But in a large aquarium or a mature tank with plenty of overgrown coral and live rock, it may be difficult to impossible to get the crab out. So what then?

Gorilla Crab Traps

Gorilla and Xanthid Crabs, once established, can be impossible to remove without taking apart your entire tank. But you can also try trapping them instead! Fortunately, one of the best ways to remove them is also one of the easiest! All it requires is a shot glass and some bait!

Once the lights are out and the fish and other day dwellers have had time to settle, grab a shot glass and something fresh and tasty, such a small piece of fresh fish or shrimp. Place the bait inside the glass and set the glass on the bottom, standing right side up. The idea is that the crab can climb up the sides of the glass and into it to eat the bait. But the sides of the shot glass are so smooth that the crab finds it impossible to climb back out. And unlike swimming crabs or shrimp, they are stuck inside permanently! A glass jar, appropriately sized, can also work if your crab is too large to get stuck inside a shot glass.

The only problem with this strategy is the other animals in your tank. You may have to try a few nights in a row, especially if you have loads of scavengers like nassarius snails, brittle starfish, and other animals that can get into the glass. But for Gorilla Crabs living in fish-only tanks or other setups with few mobile invertebrates, this approach works wonderfully!

And if you find that you can’t trap the crab? Then you’ll have to either accept that you have a Gorilla Crab or take apart the aquascape in order to get it out.

Conclusion

Live rock hitchhikers can often be beautiful. These include feather duster worms, hard and soft corals, macro algae, coralline algae, and other highly desirable organisms. But any uninspected live rock purchase can also include Gorilla Crabs, mantis shrimp, bristle worms, bubble algae, bryopsis algae, and other noxious aquarium residents.

Unless you intend to sterilize your live rock, you should always do a careful inspection of any incoming rock as best you can. While you won’t get everything, you may see an odd claw or tail sticking out where one isn’t meant to be. And when adding new live rock, take extra time to inspect the aquascape for unwanted guests. Vigilance is the best defense against Gorilla Crabs and other live rock hitchhikers!

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